The only degree of freedom that has not yet been exploited for data transmission in fiber-optic systems is the spatial dimension, which is why space-division multiplexing (SDM) is currently the subject of numerous investigations. Space-division multiplexing includes not only the transmission of data in one fiber having a plurality of fiber cores, but also the transmission of data in one fiber core conducting a plurality of propagatable modes, or a combination of both types of transmission. Coupling light into and out of such fibers has emerged as a particular problem for both types of space-division multiplexing.
The publication by W. Klaus et al. entitled “Free-space Coupling Optics for Multi-core Fibers”, IEEE Summer Topicals 2012, describes a free-space coupling device for coupling a plurality (up to 19) of standard single-mode fibers to a 19-core fiber. This solution provides a high level of efficiency, but it is easy to see that it is very complex, as well as cost-intensive.
An integrated optical system may be of assistance here: C. Doerr et al., in the publication entitled “Silicon photonic integrated circuit for coupling to a ring-core multimode fiber for space-division multiplexing”, ECOC 2011, shows an integrated optical circuit which can excite different modes in multimode fibers with the aid of a grating coupler, by using a circular 1D grating coupler with many input/output waveguides fed with different phases, with the result that ring modes are excited in the fiber. This solution is elegant, but requires a large chip area and generates only the rather uncommon ring modes in only one polarization.
The publication entitled “Space-division multiplexing and all-optical MIMO demultiplexing using a photonic integrated circuit”, OFC 2012, likewise by C. Doerr et al., reports on an arrangement of three 2D grating couplers which, when properly arranged, are capable of generating a plurality of fiber modes simultaneously. In contrast to the previous arrangement, the LP fiber modes are generated in this case in both polarizations. However, the arrangement is disadvantageous in that the three coupling gratings must radiate simultaneously with the correct amplitude and phase onto the fiber core, which either involves a very large fiber core and concomitantly unfavorable properties when propagating the modes along the fibers, or requires very small and thus inefficient grating couplers. As can be seen from the publication, the grating couplers generate the desired fiber modes only approximately, which means that some of the Input power is either lost, or crosstalks to other fiber modes.